In order to improve an image quality, an image processing device, for example has carried out a process for sharpening an image (hereinafter referred to as a sharpening process). For example, a conventional television receiver carries out a contour compensation for steepening a rise and a fall of an image signal which correspond to contour parts of an image to be displayed therein. The contour compensation is carried out by extracting a high-frequency component of an image signal (a luminance signal) supplied to a display of the television receiver, amplifying the extracted high-frequency component, and adding the amplified extracted high-frequency component to the supplied image signal. This improves a frequency characteristic of the image signal which deteriorates due to a process carried out in each circuit of the television receiver, so that an apparent image quality is enhanced.
The following description discusses, with reference to FIG. 19, an example of a frequency spectrum of an image signal which has been subjected to the sharpening process by use of a conventional technique for carrying out the contour compensation described above. (a) of FIG. 19 is a schematic view illustrating a frequency spectrum of an image signal having a sampling frequency fs. (b) of FIG. 19 is a schematic view illustrating a frequency spectrum of an image signal obtained by carrying out the sharpening process by use of the conventional technique with respect to the image signal having the frequency spectrum illustrated in (a) of FIG. 19, the conventional technique being a technique for carrying out the contour compensation described above.
As described above, a high-frequency component is added to an image signal which has been subjected to the sharpening process by use of the conventional technique. This causes an increase in frequency component in the vicinity of a Nyquist frequency fs/2 which is half of the sampling frequency fs (see (b) of FIG. 19).
Normally, the sharpening process by use of the conventional technique (described above) is a process for carrying out a linear operation with respect to an image signal. Therefore, a frequency component higher than the Nyquist frequency (a high-frequency component which is not contained in an image signal to be processed) cannot be used in the sharpening process by use of the conventional technique. This prevents improvement in image quality especially in a case where an enlargement process is carried out with respect to an image.
For example, the following description discusses a case where an image whose resolution is half of a full high-definition is subjected to an enlargement process and then displayed in a display of a television receiver of a high definition television (HDTV) (having a full high-definition of 1080×1920 pixels). Assume here that the enlargement process is a process for horizontally doubling the number of pixels by upconverting the image signal having a frequency spectrum illustrated in (a) of FIG. 19. In this case, a new sampling frequency (described as Fbs) of the image signal which has been subjected to the enlargement process is twice the sampling frequency fs (Fbs=2fs).
The following description discusses, with reference to FIG. 20, a frequency spectrum of the image signal which has been subjected to the enlargement process. FIG. 20 is a schematic view illustrating a frequency spectrum of the image signal which has been subjected to the enlargement process. There exists no frequency component between the Nyquist frequency fs/2 with respect to the sampling frequency fs and the Nyquist frequency Fbs/2 (=fs) with respect to the new sampling frequency Fbs (see FIG. 20). Same applies to a space between Fbs/2 and 3fs/2.
Accordingly, even if the sharpening process by use of the conventional technique is carried out with respect to the image signal which has been subjected to the enlargement process, it is impossible to extract a frequency component in the vicinity of the Nyquist frequency Fbs/2 which frequency component is a high-frequency component of the image signal which has been subjected to the enlargement process.
Consequently, even if the sharpening process by use of the conventional technique is carried out with respect to the image signal which has been subjected to the enlargement process, an image which has been subjected to the enlargement process is displayed in a blur.
In contrast, Non Patent Literature 1 and Patent Literature 3 disclose techniques for obtaining a high-resolution image by use of, for example, inter-frame or intra-frame autocorrelation also in a case where an image is subjected to an enlargement process.
Non Patent Literature 2 discloses a technique for obtaining a high-resolution image by use of a non-isotropic diffusion filter, the non-isotropic diffusion filter carrying out smoothing processes which are different in degree between a tangential direction and a perpendicular direction of an edge included in an image.
Patent Literature 1 discloses an image processing device which in order to improve an image quality by emphasizing a high-frequency component, uses a nonlinear circuit for adjusting each of a coring amount, a clipping amount, an enhancement amount, a limit amount, and the like of a signal to add to an image signal.
Patent Literature 2 discloses an image quality compensation circuit which in order to compensate for a high-frequency signal without lowering a video quality, uses a nonlinear conversion circuit for preventing edge parts of a pulse waveform and a step waveform of an image signal from being linked.